All humans yawn. So do most vertebrate animals. Surely it serves some useful function. But what that might be has puzzled scientists throughout the ages.

Now a series of experiments suggests a surprising reason for yawning. It cools the brain, says Andrew C. Gallup, PhD, a postdoctoral research associate at Princeton University.

"We have collected data on rats, parakeets, and humans. All the data supports the brain-cooling hypothesis," Gallup tells WebMD.

Here's the basic idea:

When you start to yawn, powerful stretching of the jaw increases blood flow in the neck, face, and head.

The deep intake of breath during a yawn forces downward flow of spinal fluid and blood from the brain.

Cool air breathed into the mouth cools these fluids.

"Together these processes may act like a radiator, removing [too hot] blood from the brain while introducing cooler blood from the lungs and extremities, thereby cooling [brain] surfaces," Gallup says.

To answer skeptics, Gallup has laid out a more detailed anatomical description of the process in the medical literature.

We Yawn More When It's Cool

Gallup's theory predicts that colder outside air should cool the brain better than hot air. The body should therefore yawn more when the air is cool, and yawn less when the air is hot.

Where better to test this than in Tucson, Ariz.? Gallup's team went there twice: Once in the winter, when it was a cool 71.6 degrees F outside, and once in early summer, when it was 98.6 degrees F.

The researchers asked 80 pedestrians to look at pictures of people yawning. It's well known that people often yawn when they see others yawn.

Sure enough, in the cooler weather 45% of people yawned when they looked at the pictures. But in hotter weather, only 24% of people yawned. Moreover, people yawned more if they'd been outside longer in the cool weather, and yawned less if they'd been outside longer in the hot weather.

These results mimicked an earlier study in which Gallup's team showed that budgie parakeets yawned more in cool temperatures than they did in hot temperatures. And it supported a rat study in which rat brains cooled a bit when the animals yawned.

What We Say When We Yawn

Gallup says his brain cooling theory of yawning is the only theory that explains all these experimental results. But he has not yet convinced those who prefer another theory.

University of Geneva physician Adrian G. Guggisberg, MD, agrees with Gallup that changes in room temperature can trigger yawning. But he's wary of the brain cooling theory. And he offers an alternative interpretation of Gallup's Tucson study.

"The fact that yawning is suppressed during high temperatures suggest that it fails precisely when we need it," Guggisberg tells WebMD. "There are other [ways to regulate body temperature], such as sweating, and it is unclear why we would need another regulator which fails when it matters."

Yawn theorists split into two camps. Like Gallup, one side says yawning must have a physiological cause, and a physical benefit. The other side says yawning is a form of communication that offers various social benefits.

Guggisberg prefers the social theory of yawning. He sees the physiological effects of yawning as too small to account for their persistence through evolution. But he sees the contagious effect of yawning as a key clue.

"The more people are susceptible to contagious yawning, the better their social competence and empathy," Guggisberg says. "In humans it is clear that yawning has a social effect. It is probably an unconscious behavior. It is not clear what yawning communicates or what it achieves. But clearly it transmits some information that has some effect on brain networks or behavior."

Across cultures, Guggisberg says, the yawn is understood as a sign of sleepiness and boredom. The yawn thus communicates to others that one is experiencing a moderately unpleasant experience but not an immediate threat.

"We might have to get used to the idea that yawns have a primarily social rather than primarily physiological effect," Guggisberg and colleagues wrote in a recent article.

Gallup argues that whatever message yawning communicates, it is far too ambiguous and subtle to be so well-conserved throughout evolutionary history.

"It is not that I don't think there is any social function to yawning, because clearly it is contagious," Gallup says. "But we have to think of it as a process driven by physiological triggers we are unable to control. If it happens in a meeting, it should not be a sign of disrespect or insult."

The Gallup study appears in the Aug. 28 online issue of Frontiers in Evolutionary Neuroscience.